Carbazole-based donor materials with enhanced photovoltaic parameters for organic solar cells and hole-transport materials for efficient perovskite solar cells.

Five carbazole-based donor molecules are designed by structural engineering of reference molecule PF. The molecules are devised by substitution of thiophene bridged end-capped acceptor groups namely (2-methylenemalononitrile) PF1, (methyl 2-cyanoacrylate) PF2, (3-methyl-5-methylene-2-thioxothiazolidin-4-one) PF3, (2-(3-methyl-5-methylene-4-oxothiazolidin-2- ylidene) malononitrile) PF4, and (4-(5-methylthiophen-2-yl) benzo[c] [1, 2, 5] thiadiazol) PF5. A DFT investigation was performed at the selected DFT functional MPW1PW91/6-31G (d,p) to investigate the optoelectronic properties of PF and all designed (PF1-PF2) molecules. Several important characteristics, i.e., band gap (E), transition density matrix analysis, dipole moment (µ), density of states analysis, reorganization energies, open circuit voltage (V), and fill factor, were investigated. The comparison of energy levels of reference molecule and designed molecules unveils the fact that these molecules are efficient hole transport materials to be used in perovskite solar cells (PSCs). All the newly drafted molecules (PF1-PF5) show higher λ values in solvent (Chlorobenzene) ranging from 529 to 614 nm than the reference PF (344 nm). Smaller band gap (E) values in a range of 2.27-1.9 eV for newly designed molecules are observed which are very much reduced when compared to reference PF. Lowered exciton binding energies (E) and reorganization energies for the electron (0.004279-0.0103337 eV) as compared to PF reveal that our molecules display higher electron mobility rates, and hence, these small molecules can be used as proficient donor materials in high-performance organic solar cells (OSCs) and better hole transport materials (HTMs) for possible application in perovskite solar cells.